Percussion welding is a well-known arc welding process for joining metal workpieces in which very intense but extremely brief and localized heat is obtained from an arc produced by a rapid discharge of electrical energy, and force is percussively applied during or immediately after the electrical discharge to impact the workpieces together. A shallow layer of metal on the contact surfaces of the workpieces is melted by the heat of the arc. Upon the workpieces being impacted against each other, the arc is extinguished, expelling molten metal and completing the weld.
Close control of parameters such as welding current and impact velocity are important for producing a good weld. The welding current amplitude and pulse shape may be controlled by a variety of methods such as varying the amount of energy stored in the system. Another method is to control the start and duration of a portion of the current cycle provided by an A.C. welding transformer. These methods determine the heating capacity of the arc. The impact velocity is related to the mass of the moving workpiece and the workpiece clamping member of the machine, and determines the amount of forging force. The forging force must be great enough to accelerate one of the workpieces being welded to a high velocity within the short gap of the machine, and is generally adjusted empirically until the desired weld quality is obtained. Conditions are usually adjusted to give the shortest arc time that will permit consistent production of welds with desired properties. If the parts being welded are forced together too soon, the arc will be extinguished before the work surfaces of the parts are melted. If the impact is delayed too long after arc initiation, the melted interfaces may solidify too soon to permit expulsion of excess molten metal. In both cases, poor fusion and bonding of the parts results.
It is important to be able to determine reliably the quality of the welds produced by percussion welding to detect defective welds, particularly in an automated process where parts are fed automatically to the welding machine. Experienced operators can often tell by the sound produced during welding that a weld is defective. However, this is not a very reliable approach and is not practical for an automated process. Other known approaches for detecting defective welds have involved monitoring the time-varying welding current and comparing the current time duration or amplitude characteristics to preselected criteria. While such current monitoring techniques are capable of detecting a substantial number of defective welds, they have a number of disadvantages.
It is desirable to provide techniques for detecting defective welds in real time and at high speed which are more sensitive and more reliable, and which are readily adaptable to an automated welding process. It is to this end that the present invention is directed.